The present invention relates generally to a system for detecting fractures and corrosion in a workpiece. More particularly, the present invention relates to a unique device and method for monitoring the change in electrical properties in a thin film wiring layer, which changes are indicative of a fracture or corrosion of the wire caused by a fracture or degradation of the workpiece. The present invention allows maintenance personnel to easily diagnose and pinpoint fractures and corrosion in workpiece, thereby facilitating repair thereof.
Large and/or unusually shaped workpieces employed in the aerospace industry are typically visually inspected to detect cracks or fractures. Such inspections are usually performed at regular time intervals as part of a conventional time-based maintenance (TBM) program. Fractures which occur between inspection intervals can enlarge to a dangerous extent before detection. For example, fractures along the bulkhead or engine mounts of an aircraft may be repaired without hindering the performance of the aircraft if the fractures are detected while still relatively small. However, in order to visually inspect many sub-components, costly disassembly of the workpiece may be required. Because disassembly only occurs on a periodic basis, inspection often fails to uncover minor fractures before the workpiece suffers additional structural damage.
In order to overcome the problems associated with periodic visual inspection programs, fracture detecting systems have been devised to periodically inspect the surface of a workpiece to identify any fractures in the workpiece. For example, eddy current and radiography fracture detection methods have been utilized. In addition, ultrasonic fracture detection systems have been developed which typically only detect fractures in the line of sight between the ultrasonic transmitter/receiver. Each of these known systems require time-consuming, large area scanning by a highly trained technician which is time consuming and very costly. Furthermore, because these inspections are still performed on a periodic basis, fractures within workpieces of a complex shape may not be detected for a substantial time period, allowing them to enlarge to a size which can cause the workpiece to fail.
In order to overcome the problems associated with periodic inspection of a workpiece, an optical fiber fracture sensing system was developed and disclosed in U.S. Pat. No. 4,636,638 (hereinafter the "638 patent"). The sensing system disclosed in the '638 patent includes an optical fiber mounted on stress risers on the surface of the workpiece. The present inventor's own previous invention, disclosed in U.S. Pat. No 5,525,796 issued on Jun. 11, 1996, entitled FIBER OPTIC SENSING APPARATUS FOR DETECTING A FRACTURE IN A METALLIC WORKPIECE AND AN ASSOCIATED METHOD OF ATTACHING A FIBER OPTIC SENSING ELEMENT TO THE METALLIC WORKPIECE, assigned to the assignee of the present invention and herein incorporated by specific reference thereto, discloses an improved fracture detecting fiber optic element securely affixed to the workpiece with a metallic material, such as a metallic solder.
While optical fiber detection systems have the advantage of providing continuous monitoring of a workpiece, it has proven difficult to apply optical fiber detecting systems to workpieces having complicated or unusual shapes. For example, a helicopter rotor strap pack is a component having a complicated shape necessary for attaching the rotor blade to the rotor hub. The strap pack is normally contained within the blade pitch bearing housing and, in the AH-64 Apache helicopter, consists of 22, 20 mil thick laminates. Because of the shape of the strap pack, fiber optic inspection is not possible. As an alternative, an attempt has been made to use active and passive acoustic emission systems to "listen" for cracks. This method is almost physically impossible due to the fact that the strap pack is comprised of a plurality of laminated metallic strips with air gaps between each piece. High frequency sound waves are prevented from crossing the gaps due to the tremendous impedance mismatch. As a result, there is currently no way short of disassembly of the strap pack from detecting cracks on the various straps.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a system capable of continuously and/or periodically monitoring a complex workpiece such as a strap pack for fractures without the need for costly disassembly and visual inspection. The successful solution must be able to periodically or continuously monitor the workpiece for fractures without the need for disassembly.